1. Field of the Invention
The present invention generally relates to photosensing techniques and, in particular, to a system and method for controlling charge transfers for photosensitive arrays.
2. Related Art
Image scanners convert a visible image on a document or photograph, or an image in a transparent medium, into an electronic form suitable for copying, storing or processing by a computer. An image scanner may be utilized as a part of a camera, a copier, a facsimile machine or other type of device that captures images during operation. In order to capture an image, light is typically reflected off the surface of a document, through an optics system, and onto a photosensitive device.
Each photosensitive device has one or more photosensor arrays, and each photosensensor array typically has thousands of individual photosensitive elements. Each photosensitive element, in conjunction with the scanner optics system, measures light intensity from an effective area on the document thereby defining a picture element (pixel) on the image being scanned.
In measuring light intensity from an effective area on the document being scanned, each photosensitive element within a photosensor array collects charge based on the intensity of light received by the photosensitive element. After collecting charge for a specified time period, referred to as an “exposure period,” the charges within the photosensitive elements are transferred into a charge transfer register. Usually, charges from different photosensitive elements are transferred into different cells of the charge transfer register such that all of the charge in any cell defines the measured light intensity for the same pixel or, in other words, are transferred from the same photosensitive element.
Once the charges are transferred into the charge transfer register, the photosensitive elements begin collecting charge for the next exposure period. Usually, the charges in the cells of the charge transfer register are serially shifted out of the charge transfer register “bucket-brigade” style. Each set of charges shifted out of the charge transfer register may be converted into an electrical signal, which is later used to define a color value for the corresponding pixel of the image being captured.
Due to various synchronization and data reliability constraints, situations arise when it is desirable for a sample of a photosensor array to be based on an exposure period having a length of time different than the selectable lengths. In such a situation, the exposure period having a time length closest to the desired time length is selected. However, a slight difference may exist between the selected time length and the desired time length.
Some photosensor arrays address the foregoing problems by employing a shutter that effectively enables a photosensor array to take a sample of any desired exposure time length. However, the shutter adds a relatively significant amount of structure to the photosensor array, thereby significantly increasing the size and/or cost of the photosensor array.